/**
 * Definition for singly-linked list.
 */
struct ListNode {
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};


class Solution {
public:
    // Time: O(n); Space: O(1)
    ListNode* reverseList(ListNode* head) {
        ListNode* prev = nullptr, * next = nullptr;
        ListNode* curr = head;
        while (curr) {
            next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }

    // 反向工作：nk->next->next = nk;
    // n1->next = nullptr;
    // Time: O(n); Space: O(n)
    ListNode* reverseListRecursive(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode* newHead = reverseListRecursive(head->next);
        head->next->next = head;
        head->next = nullptr;
        return newHead;
    }
};
